1. Field of the Invention
This invention relates to the field of vertically panning pictures in television receivers, and in particular, to vertically panning enlarged pictures from conventional source material in wide screen television receivers, and the like.
2. Description Of Related Art
The ratio of the width to the height of the borders of a picture or the borders of a display screen for a picture are referred to herein as the format display ratio. The ratio of the width to the height of images forming a picture is referred to herein as the image aspect ratio. A mismatch of format display ratios which results in distorted images in a picture is referred to herein as image aspect ratio distortion.
Conventional source material having pictures with a format display ratio of 4.times.3, for example, cannot be displayed on a wide screen television receiver with a format display ratio of 16.times.9, for example, unless the video signal is speeded up. For these format display ratios, for example, the video signal must be speeded up by a factor of 4/3. This results in 1/4 of the display screen being blank, which can be deemed by viewers as an undesirable operating condition, in which the wide feature of the television receiver is wasted.
An alternative to speeding up the video signal is to vertically enlarge the picture. This can be accomplished by manipulating the vertical deflection height or by vertical interpolation of the video signal. If the vertical height of a picture having a 4.times.3 format display ratio is increased by a factor of 4/3 on a wide screen display having a format display ratio of 16.times.9, without speeding up the video signal, then the picture will have no image aspect ratio distortion as displayed, and in fact, the picture, as defined by each image of the picture, will be larger in size. Moreover, the screen will be completely filled by a picture. However, 1/4 of the picture will be missing. If the picture happens to be centered as enlarged, or zoomed, then the top 1/8 and bottom 1/8 will be cropped, that is, missing. This cropping presents no problem if the picture is also in letterbox format, wherein approximately the top 1/8 and approximately the bottom 1/8 of the picture are dark bars having no information content. In fact, this is a very desirable display mode for a wide screen receiver.
However, if the source picture is not in letterbox format, then 1/4 of the picture content will be missing. The center of action may very well be cropped. Under these circumstances, it is desirable to vertically pan the zoomed picture, as necessary, to follow the center of the action. A vertical panning capacity enables viewer choice as to which part of the zoomed picture will be shown and which part will be cropped. Vertical panning can also be implemented responsive to a panning control signal transmitted with the video signal, for example, which requires an appropriate decoder in the receiver.
A first analog approach for vertically positioning a picture is superimposing a DC component on the vertical deflection current. This requires a DC coupled vertical deflection amplifier and sufficient output current range. Disadvantageously, such an amplifier suffers increased dissipation losses.
A second analog approach uses a floating DC current source coupled in parallel with the vertical deflection yoke. The dynamic range of the output voltage of the current source must be large enough to account for the flyback pulse.
A serious problem with all approaches using current superposition is that the vertical S-correction and the East-West-correction must be automatically adjusted over the entire range of the vertical shift.
A third analog approach adapted for use with horizontal deflection systems operating with interlaced fields at f.sub.H, designating the basic horizontal synchronizing frequency, provides for phase shifting of the vertical deflection with respect to the video signal using analog pulse delaying techniques. In one such known circuit three one-shot multivibrators are connected in series, and provide respective pulses having durations of T1, T2 and T3 respectively. A vertical drive output pulse is an input to the first one-shot. The first two one-shots generate a total delay time of T1+T2. The output of the third one-shot is a delayed vertical drive output pulse having a duration T3. A shift control input signal can be used to vary the duration of T2, the output pulse of the second one-shot, between a minimum value and a maximum value. This results in shifting the picture downwardly and upwardly respectively. Such analog circuits have serious problems, causing incorrect interlacing and jitter.
Vertical panning circuits utilizing digital techniques have been developed for use with horizontal deflection systems operating with non interlaced fields at 2f.sub.H, that is, twice a basic horizontal synchronizing frequency f.sub.H.
In a first digital approach, a panning control circuit adjusts a vertical blanking interval in phase relative to the vertical synchronizing component to control which portion of the enlarged picture area is displayed and which portion is not displayed. This system requires a DC coupled vertical deflection system in order to function properly. As a result of the DC coupling, implementation of a vertical zoom results in the bottom part of the picture being cropped, absent any vertical panning. Some vertical panning is required in order to center the picture, which is usually deemed necessary as a starting point for a vertical zoom display mode.
A circuit for automatically adjusting S-correction which can be used with the first digital approach described above can utilize a differential amplifier formed by a pair of transistors, which couple a vertical sawtooth signal to an input side of a vertical deflection amplifier. Nonlinearity of the transistor pair provides linearity or S-correction in the vertical direction. The amplitude of the sawtooth signal is adjusted by adjusting a vertical height control signal. The vertical height control signal is coupled to the emitters of the transistor pair for controlling the nonlinearity, in order to compensate for the change in the S-correction requirement introduced by the amplitude adjustment.
In a second digital approach, referred to as a frame based vertical panning system, the panning control circuit counts the horizontal lines, or half lines, and generates a vertical reset signal delayed by a variable pan delay relative to the vertical synchronizing component of the video signal. The pan delay can vary in mutually exclusive ranges to control interline flicker of the panned video signal when successive fields have different numbers of horizontal lines, such as during the pause mode of VCR playback wherein the number of lines per field varies as a function of tape speed and recording mode.
In a third digital approach, referred to as a field to field vertical panning system, is based on a video signal having a standard field length and having a vertical synchronizing component dividing groups of successive horizontal lines into successive fields. The panning control circuit measures a variance between the actual length of each successive field and the standard length. A vertical reset signal is delayed by a pan delay relative to the vertical synchronizing component of the video signal. The pan delay can be adjusted on a field by field basis responsive to the field length variance, if any, to control interline flicker of the panned video signal when the fields have nonstandard field lengths.
A clear need is established for a vertical panning circuit which is can be used with a television receiver which operates at f.sub.H, which does not cause interlacing problems, which can be used with both AC and DC coupled vertical deflection systems, which does not have increased dissipation losses and which does not require continuous readjustment of S-correction and East-West-correction during vertical panning.